How to Control Johnson Grass

Expert Reviewed
Johnson grass is an invasive grass species that can grow alongside a lot of popular crops. If Johnson grass isn’t controlled, it can kill most of those crops or at least decrease crop production. You can prevent the grass from spreading by plowing immediately after you harvest, till frequently, and use Johnson grass-free seed and feed. You can also use herbicides to kill already growing Johnson grass. Finally, you can control Johnson grass growth in your lawn by removing the plants by hand and monitoring the area.

Preventing Johnson Grass from Spreading

Plow immediately after you harvest. Johnson grass has rhizomes that allow the weed to spread into uninfected areas. If you plow your fields immediately after you harvest your crops, it breaks up the rhizomes and prevents them from getting a good hold and then spreading to other areas.

Till your soil a few times per week. You should till the soil of crops where Johnson grass grows more frequently than other areas. If you till less frequently than this, it will break up the Johnson grass rhizomes, but not completely destroy them. That means that the broken-up rhizomes can eventually spread to other areas.

Mow or graze Johnson grass-infected areas closely. This means mowing or allowing your livestock to graze in areas where the Johnson grass is already taking hold. Doing this for two years eventually brings the rhizomes to the top of the soil, which prevents them from being able to spread.

This is the best option if you have Johnson grass in an area where you can't spray herbicides or till your soil.

Use Johnson grass-free seed. Some seed for crops actually has Johnson grass in it. It should be listed on the ingredient list on the packaging. Choosing a seed without Johnson grass will prevent the growth and spread of new Johnson grass.

Clean equipment used on Johnson grass to prevent spreading. If you’re using your equipment to break up Johnson grass but you don’t clean your equipment after, it can actually spread the grass. Make sure any blades are clean, as well as the bottom of any machines. You can clean them easily with soap and water.

Use Johnson grass-free livestock feed. Johnson grass can also pop up in some livestock feed. Check the ingredient list to make sure your feed is free from Johnson grass. Your livestock can spread it if they drop their food, and also from their droppings after they've eaten feed with Johnson grass in it.

Reducing Johnson Grass with Herbicides

Use post-emergence herbicides in growing crops. If you have Johnson grass in corn or soybean fields, you’ll need to wait until the Johnson grass appears, because trying to break up the rhizomes under the soil can also wreck your crops.

Use post-emergence herbicides like Beacon or Accent on 4- to 10-inch-tall (9 to 25 cm) Johnson grass seedlings in corn crops. You can also apply it as a broad treatment over your corn until the corn is 24 inches (52 cm) tall.

Use Assure II, Bugle, and Fusion post-emergence herbicides on soybean crops when the Johnson grass is less than 10 inches tall.

Spot spray infested areas. Johnson grass doesn’t grow evenly, so if you notice any growing, consider spot spraying your crops, rather than treating all of them. Mix the herbicides mentioned above with water into a 1 to 2 percent mixture (1 to 2 percent of the herbicide in the solution) and spray specific spots.

Use Roundup 7 days before you harvest your crops. You can use Roundup on most mature crops without damaging the crops themselves as long as the crops have been genetically modified to resist Roundup. A week or so before you’re going to harvest your crops, spray them with Roundup. Spraying Roundup near harvest should break up the rhizomes enough that they come up when you harvest.

Roundup is available at most farm supply stores, as well as many hardware and home improvement stores.

Removing Johnson Grass from Your Lawn

Wear gloves to remove plants by hand. If you can access some individual Johnson grass plants near the edge of your lawn, you can pull them up by hand. Wear heavy gloves since the leaves can be sharp. 19 days after you remove the plants, check for seedlings in the same area and remove those, too.

It will take Johnson grass about 19 days to put up new seedlings, so you should check the area right around 19 days after you pull up previous plants.

Spray the Johnson grass plants with 2 percent glyphosate herbicide. Before you spray the herbicide, put on waterproof gloves, long pants, a long-sleeved shirt, and a face mask. Coat each plant with herbicide, but don’t spray so much that the leaves start to drip.

Make sure you’re spraying the herbicide on a windless day. If it’s too windy, the herbicide can spread to plants where you don’t want it.

Cut down the plants once they turn brown. How long it takes will depend on the brand of herbicide you use, so make sure you check the label. Once the plants have gone brown, cut them down at the base of the plant with pruning shears.

Dig up the rhizomes once you’ve removed the plants. Using a garden hoe or rotary tiller, break up the soil for several feet out from the base of the Johnson grass plant. Johnson grass rhizomes are thick and white in color. Remove as many as possible and throw them away.

Use your tiller or pruning shears on stubborn rhizomes. You might not be able to completely remove some of the Johnson grass rhizomes. If you can't remove them, use your tiller or pruning shears to cut the rhizomes into fine pieces in the soil. This will prevent them from growing and extending.

Cut down any other Johnson grass plants that appear. After you’ve treated visible plants and cut and up and removed as many rhizomes as possible, keep an eye on your growing area. If you see new Johnson grass plants, cut them down as they appear.